I got the following error:
Error unmounting hda1: Device or resource busy
It occured after I got the db3 Invalid argument message (bug 11884) and
after it notified me that I didn't have enough disk space to install the
selected packages.

I had this with BETA1
Verified it occurs with BETA2. It is IMHO a _very_ serious bug
To reproduce
1) Custom install
2) Pick "everything"
3) Pick "choose individual packages"
4) I attempted to install to a 1.5Gb / (no other partitions)
5) It runs out of space, says I need a few hundred Mb more
6) Hit OK to "out of space" message
7) Message box appeared: "unable to unmount /dev/hdc2" /dev/hdc2 was /
8) Hit OK to error dialog
9) Hit BACK a few times and select a set of packages that fit in the space
10) Hit NEXT a few times to attempt re-install AND
11) Install dies _totally_ with "error mounting /dev/hdc2"
12) Ouch! Ouch!
NOTES
=====
- I'm not 100% sure, but I think it also goes wrong with a large / partition
and a /usr which was too small
Please mail me if it cannot be reproduced; the field will cane you guys if it
doesn't get fixed for 7.0 release ;-)

this issue has been noted and marked as a duplicate of 12106 even though
circumstances are slightly different, since the base cause is the installer not
handling insufficient space reqs more gracefully ... thank you for your report !